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It's hard for me to overstate how much I love the Technic Crawler Crane 42042. I've been constantly tinkering with it and making improvements since I got it about 6 months ago. My method is basically to lift heavier and heavier stuff until it falls over or something breaks. Then I rebuild. This is how it got to where you see it today. I don't really care about the tracks or the slewing. I want to lift heavier stuff, higher!

So far my mod/improvement list reads like this:

Upgraded the block & tackle harness for better mechanical advantage

Added a quick-adjust system for efficient rigging changes to the harness, main hoist, and hook

Beefed up gantry or A-frame: wider, taller, and much stronger

Improved existing back hitch and added a second back hitch

Counterweight box

Strong, longer boom

Paid almost (!)$20 for stupid Technic metal hook

Integrated claw storage on gantry

XL Motor upgrade

Boom foot return damper thing

Replaced LEGO string with synthetic cable (from Chin-a)

Here it is in action.

Read on for a more thorough description and photos of each mod/improvement. Click through on the photos to view them at larger sizes.

Block & Tackle
The first thing I did was add blocks to the block & tackle system allowing me to really turn up the mechanical advantage the motor has over the load when raising and lowering the boom. This of course comes at the cost of speed, but I'm OK with that. The nice thing about this is I don't have to actually use all the blocks.

Quick Adjust Rigging on Harness and Main Hoist
You can see my bright red and yellow-handled quick adjust axles with the red and yellow grips on the side of the harness assembly. This makes it easy to slip the rigging in and out of the blocks to fine tune my power/speed balance for the specific load. I also have these at the end of the boom and on the pulley so I can adjust the speed/power tradeoff on the hoist line. I added this quick disconnect system after I'd re-strung the rigging for the hundredth time.

I can re-thread the rigging on both the harness and the main hoist line to go from "high gear" with max lift speed to "low gear" for max power in just a minute or two. (I had originally made these with axles that have stoppers on the end, but ended up using regular axles, because I have a ton of them.)

A couple of timelapse (YouTube) videos of the quick change.

Stronger & Wider Gantry
The gantry in the stock model is really weak. It is wholly comprised of 4 Technic lift arms held in place with 6 friction pins. As I added heavier loads this system was quickly outgrown. I was bending lift arms and even popping the entire assembly off the superstructure. So I rebuilt. The new gantry is wider, allowing for more blocks in the harness. It is many times over more rigid and is taller, allowing the harness mount to be further back over the body, which in turn allows the main boom to be drawn up higher to a much steeper final angle.

This piece took a long time and has been through the most revisions over the months. I did a lot of research by looking at other LEGO models, other and peoples crane MOCs. Both arms of this gantry are heavily inspired by the gantry on LEGO Crawler Crane 8288 (http://brickset.com/sets/8288-1), which I would love to own a copy of someday (not today).

Improved Back Hitch
The back hitch (the two black lift arms supporting the gantry) also quickly became insufficient. The anchor it was tied into was bending and getting stress cracks, so I created a stronger attachment system for it elsewhere. I also added a second back hitch that ties directly into the improved counterweight...

Counterweight Box
One of the challenges with cranes is counterweight. As the amount of load it can lift and move improves, their is a corresponding increase in risk of tipping. The original model cleverly uses the battery box mounted all the way at the back edge of the body. It's literally the furthest element from the tip of the boom. I added a weight box behind it that is open and easily filled with ballast of my choosing. C Batteries work extremely well because of their density and modularity. It has an open design so it's easy to adjust the weight on the fly as needed depending on the load and the angle of the boom. After building it, I added a the second back hitch directly attached to the gantry to remove stress from the body connection of the weight box and from the first front back hitch (heh). When the gantry starts carrying a lot of load, I don't know that the front hitch bears any at all.

Stronger, Longer, Better Boom
Maybe the most noticeable feature is the length of the new boom. The original is 30 studs long, not counting the butt and tip, with an optional section to extend it to 45 studs. This booms main section is 75 studs long. At this length, it had some really concerning droop/bending at full extension. However, I was also able to easily reconfigure the inner supporting structure to be much stronger since I didn't care about the ability to easily add and remove sections. I also added the outer white panels to help with rigidity as they span the weak links between the blue 15L lift arms that are the main defining component of the boom.

The boom tip received an upgrade in the form of a smoother operating block and tackle system (with quick disconnect axle just like the harness) controlling the main hoist. I also added an aviation warning light, obviously the most complex and impressive mod thus far :)

Main Hook/Claw Storage
I purchased a Technic Large Metal Hook (http://www.bricklink.com/v2/catalog/catalogitem.page?P=70644#T=P) for like $15. It's easily the most I've ever paid for a single LEGO piece. Considering the time I've spent on this model, though, I thought it was worth it. It's mounted to the main hoist with the smallest of my quick adjust rigging axles. I can attach the hook directly to something of course, or I can use it to hang the functional claw the system came with.

When the claw is not in use, I have a storage peg for it that can be placed on either side of the gantry. I built this after I got tired of it sitting on the bench next to the crane most of the time. Much like the quick adjust rigging system, it's a simple mod that has a high convenience impact when moving this thing around for play, improvements, and storage.

More Power (XL Motor upgrade)
After removing the clutch gears and incrementally strengthening everything else to lift progressively heavier and heavier loads through probably hundreds of lifts at this point, the 9V Power Functions L Motor (99499) that came with the system finally died (predictably). To LEGO's credit, they're already shipping me another. They didn't even ask if it had been systematically subjected to increasing levels of abuse since purchase. Rather than wait for the replacement to arrive I hooked up an XL I had laying around. It's slower, but has more torque. As you can see it is mounted almost all the way at the front of the body, which is not ideal considering the weight. I may end up routing it further back if I can figure out a way to that looks this good. I did have to git rid of the ladder that was here on the original model, but this is Technic, so whatever. It's not like I'm bouncing minifigs around and doing voices when I play with the thing.

Boom Return Damper/Helper
If you increase the mechanical advantage of the harness too much, the boom has trouble releasing from very high angles. There just isn't enough weight to pull the cable off the spool on the base. I added this spring mechanism to the butt of the boom to help push it out when there is very little lateral load. I may be swapping these gray Technic springs out soon for the stronger yellow ones as it could be more effective. They will also look better matching the contrasting yellow parts of this assembly against the blue.

Rigging Cable
Through many rounds of adjusting and rebuilding I replaced the rigging cable several times. As I added to the superstructure more cable length was needed. Going down certain oaths and backtracking also had me replacing the cable with different lengths over and over. LEGO string is expensive, so I ended up buying a small spool of this stuff off Amazon: https://www.amazon.com/gp/product/B00LDIKURQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 after seeing it recommended elsewhere on these boards. It works well. It's thicker than LEGO string, so be aware, but it's also stronger and it has a bit of a metallic shine, so it looks more like the steel cable you'd see on the real life inspiration for this model.

Conclusion
That's about it. The major takeaway is that this thing can now lift 750 grams up off the floor below my bench and place it on the bench right next to it (from up to 80 cm away). Good stuff. I also think I retained most of the good looks of the original set. I'm not done tweaking yet, but I wanted to do this write up and show it off a bit before I break it again, because my next mod is likely going to happen in the gear box, if at all.

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Love the modifications and the detailed pictures! Did you buy a second 42042 to source the parts for most of the mods (except the obvious new stuff like springs and yellow parts)? I plan to mod my 42042 similarly but have been distracted by some GBC C modeling using 42042 for the last few months. ????